An inkjet image forming apparatus, such as a printer, a facsimile machine, a copier machine, or a plotter, comprises an inkjet head to eject ink droplets to record images. The inkjet head has nozzles through which ink droplets are ejected, ink flowing paths usually including chambers for applying pressure to the ink therein and in communication with the corresponding nozzles, an ink feeding path, and so on, and portions for creating and applying pressure to the ink in the flowing paths to eject ink droplets. There are also devices for ejecting other kinds of liquid droplets, for example, those ejecting droplets of liquid resists, and those ejecting droplets of DNA samples.
In an inkjet head, various methods are employed to apply pressure to the ink in an ink flowing path so as to form droplets of ink and eject them. The following methods are well known in the related art.
Japanese Unexamined Patent Publication No. 2-51734 discloses an inkjet head in which an electromechanical transducer, for example, a piezoelectric element (a piezoelectric crystal), is used as a vibrating plate to form a vibrating wall of a chamber to apply pressure to the ink (the pressure application chamber). When the crystal receives a charge, the vibrating wall deforms and vibrates, therefore changing the volume of the ink chamber and forcing some of the ink in the chamber out through the nozzle. This is the so-called “piezoelectric inkjet head”.
In addition, Japanese Unexamined Patent Publication No. 61-59911 discloses another inkjet head in which a resistor is used in each pressure application chamber to create heat; this heat vaporizes the ink in the chamber and creates a bubble. As the bubble expands, some of the ink in the chamber is pushed out by the pressure. This is the so-called “thermal inkjet head”.
Further, Japan Unexamined Patent Publication No. 6-71882 discloses still another inkjet head in which an electrode is placed facing a vibrating plate that forms a wall of each pressure application chamber. Because of the electrostatic force created between the electrode and the vibrating plate, the vibrating plate deforms and vibrates, therefore changing the volume of the chamber and forcing some of the ink in the chamber out through the nozzle. This is the so-called “electrostatic inkjet head”.
The inkjet heads mentioned above exhibit two kinds of methods of forming ink droplets. In one of them, a vibrating plate is pushed inward relative to the pressure application chamber, decreasing the volume of the chamber, and forcing some of the ink out. In the other method, a vibrating plate is pulled outward relative to the chamber and thus expanding the volume thereof; then the vibrating plate deforms so as to recover from the expanded shape to its original shape, and therefore, forces some of the ink out.
In an inkjet head using the second method (pulling the vibrating plate), as an initial state, a bias voltage is applied to the piezoelectric element to charge the element. Then the piezoelectric element discharges (releases the stored charge), leading to contraction of the piezoelectric element. Accordingly, the volume of the chamber increases, and this pulls more ink into the chamber from outside, for example, the ink feeding channel. Then, a driving signal is applied to the piezoelectric element to charge the element rapidly, causing rapid expansion of the element, and this rapidly decreases the volume of the chamber and forces some ink droplets out through the nozzle.
Next, with reference to FIG. 12 and FIGS. 13A through 13H, explanations are made of the operations of a head control device for controlling an inkjet head that employs the second method to form three kinds of ink droplets (referred to as “dots” below) by using the d33 mode of a piezoelectric crystal.
FIG. 12 shows a head control device of the related art.
In the head control device shown in FIG. 12, a driving signal Vcom including a number of driving pulses (shown in FIG. 13) is output from a driving signal generator 101, and is input to a piezoelectric element 103 through a switch 102. The switch 102 is switched ON or switched OFF depending on the output signals of a decoder 104 through a level shifter 105.
The decoder 104 includes gate circuits 110 through 112, which receive recording data signals L0, L1, L2 stored in a not-shown memory and gate signals M0, M1, M2, respectively, whose levels are controlled within a recording period so as to select the desired recording data, and an OR circuit 113 that sends the signals from the gate circuits 110 through 112 to the level shifter 105.
Here, it is assumed that a small dot is to be formed when L0=1, a medium dot is to be formed when L1=1, and a large dot is to be formed when L2=1; further, when L0=L1=L2=0, the piezoelectric element will not operate and no dots are formed.
FIGS. 13A through 13H show timing charts of signals in the head control device in FIG. 12 when operated to form the above dots. Specifically, FIGS. 13A through 13H show waveforms of the driving signal Vcom, signals selected from the driving signal Vcom and applied to the piezoelectric element, and the gate signals M0, M1, M2.
When a large dot is to be formed, that is, when L2=1, by setting M2=1 in the period from the time T10 to the time T11 as shown in FIG. 13F, a driving pulse for forming a large dot as shown in FIG. 13B is extracted from the driving signal Vcom and applied to the piezoelectric element 103.
In addition, when a medium dot is to be formed, that is, when L1=1, by setting M1=1 in the period from the time T11 to the time T12 as shown in FIG. 13G, a driving pulse for forming a medium dot as shown in FIG. 13C is extracted from the pulses in the driving signal Vcom and applied to the piezoelectric element 103.
When a small dot is to be formed, that is, when L0=1, by setting M0=1 in the period from the time T12 to the time T13 as shown in FIG. 13H, a driving pulse for forming a small dot as shown in FIG. 13D is extracted from the pulses in the driving signal Vcom and applied to the piezoelectric element 103.
In this way, by generating a common driving signal Vcom including driving pulses for forming various kinds of dots, selecting appropriate driving pulses from the driving signal Vcom according to the predetermined gate signals and recording data signals to switch ON or switch OFF the appropriate channel, and applying the selected driving pulses (waveform) to the piezoelectric element, ink droplets of different sizes, in other words, dots of different grade levels, can be formed with a single driving signal Vcom.
In the above process, before a piezoelectric element is operated, it is preferable to apply a bias voltage to the piezoelectric element in advance to keep the piezoelectric element in a charged state (expanded condition). As described above, this charged state is the initial condition of the piezoelectric element when it is operated to form ink droplets. For example, this treatment is necessary for a piezoelectric element not required to form a dot in the present recording period. Additionally, even for a piezoelectric element that has been operated to form a dot in the present recording period, it is still preferable to keep the piezoelectric element in a charged state before the next driving pulse is applied.
However, in a head control device having the above configuration, for example, considering a piezoelectric element not forming a dot in the present recording period, even then a bias voltage is applied in the preceding recording period and the piezoelectric element is kept in a charged state by that bias voltage. Because of natural discharge of the piezoelectric element, the potential of the element decreases in the present recording period.
Due to this, when a driving pulse for ejecting an ink droplet is applied in a following recording period, because the potential right before ejecting is too low, it is difficult to form an ink droplet containing a desired amount of ink.
In the same way, for a piezoelectric element that has been operated to form a dot in the preceding recording period, in the duration in which a driving voltage is not applied to the piezoelectric element, the natural discharge occurs. If this duration before a driving voltage is applied is long, the potential of the piezoelectric element decreases noticeably because of the natural discharge; consequently, even if a desired driving pulse for ejecting a desired ink droplet is selected and applied in the present recording period, since the potential right before ejecting is too low, it is difficult to form an ink droplet containing a desired amount of ink.
As a solution to this problem, Japanese Unexamined Patent Publication No. 2001-10035 discloses an inkjet recording apparatus in which, at a specified timing in each recording period, a bias level is selected from the driving signal to re-charge the piezoelectric element to the bias level.
However, in the above inkjet recording apparatus, a time interval related to the re-charging level has to be allocated in the driving signal. The length of the time interval is determined taking the reaction time of a switch into consideration, that is, the duration from the time when a switching command is issued to the time when the switch is actually switched ON or switched OFF. Usually, the time interval should be set relatively long.
However, in order to increase image formation speed, it is desirable to make the ink droplet ejection period short, so it is difficult to secure an additional re-charging time period that is irrelevant to ink droplet ejection operation. Further, in order to increase the number of the grade levels to improve image quality, it is required to allocate more pulses in the driving signal, and this also makes it difficult to secure the additional re-charging time period in the driving signal.